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Bluing/blueing is definitely a legitimate word, though may not be commonly used for the situation you describe.

This term (US Eng: "bluing"; UK/commonwealth often "blueing") is also used in the firearms and steelwork worlds, as a reaction (the formation of surface magnetite), a substance to emulate that reaction (various chemical compounds, typically based around selenium dioxide), and as the verb to describe the process of applying such a substance or reaction to steel, and to describe the color change as the effect takes place.

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